Streaming Media

1. Streaming Media

2. Unicast • Redundant traffic • Multicast • One to many

3. Video Multicast • Multicast Support • multicast enabled network • Real-time Requirements Support

4. Supporting Real-time Requirements • QoS and Resource reservation • Resource Reservation to bound data delivery delay, loss, jitter etc. • Adaptive Rate Control • Adjust video traffic characteristics to suit the Internet.

5. Multicast • Group Addressing • Distribution Tree • Join(graft) • Leave(prune)

6. Video Traffic • Periodic generation of frames at regular intervals. • Variable bit rate. • Frame periodicity must be maintained for the video to appear “smooth” • Data unavailable at playout is useless. • Jitter (variability in interarrival times)

7. Buffering and Start-up Latency • Congestion leads to Data Loss • Decreasing Data Rate • Error Control • Summary • Delay Sensitive, Loss Insensitive

8. Multicast and Heterogeneity • The Internet is Heterogeneous • Infra-structural (Spatial) • Traffic density (Temporal) • Administrative • Fairness Goal • Every receiver should receive video that is commensurate to the resources available. • Is this fair to “other” traffic?

9. Fairness • Intra-session fairness • Inter-session fairness

10. Rate Adaptive Digital Video • Compression, Scene Complexity, Motion Feedback Smoothing Buffer Video Encoder Network

11. Raw video stream is fed to encoder • Encoder sends encoded data to buffer • Buffer level provides feedback • Feedback is used to control data output rate at encoder. • Quantization, frame rate, pixel resolution etc. are controlled.

12. Network feedback can also be used. • Queueing information (internal to the network) • End-system information

13. Adaptive Bit-Rate Video • Single Stream Adaptive Approach • Replicated Streams Adaptive Approach • Layered Video Streams Approach

14. RTP • Real-Time Transport Protocol • End-to-end Protocol • NO notion of “Connection”. (hence unreliable) • Application level • Requires framing and segmentation be taken care of by lower layers.

15. RTP (continued) • Divided into two parts (consecutive ports for UDP) • Data (audio + video packets, even-numbered port) • Control • Can use single PDU in case UDP is not used.

16. Real-time Transport Protocol

17. RTP Data Packets • 12 byte header • data (video/audio) • can be encapsulated in encoding-specific layer.

18. RTP Data Packet Header • Payload Type (1 byte) • eg: JPEG etc. • Timestamp (32 bits) • generation instant of the data • Sequence marker (16 bits) • packet seq. number to help loss detection • Marker bit • end of frame for video • beginning of talk-spurt for audio

19. RTP Data Packet Header (contd) • Synchronization Source Identifier (32 bits) • randomly generated identifying session source.

20. RTP Control Channel • Control protocol called RTCP • QoS monitoring and Congestion Control • multicast • all other receivers know how others are doing • sender-report, helps receivers compute data-rate • Intermedia Synchronization • wall-clock time + RTP timestamp • allows synch of audio and video • Identification • Detailed identification of participant instead of just a 32 bit identifier. • Session size estimation and scaling • scaled to 5% of data rate

21. RTCP Packets • Several types to carry a variety of information • Source description (SDES) • CNAME, email, location, name, ... • Sender report (SR) • Bytes sent -> estimated rate • Timestamp -> synchronization • Receiver report (RR) • Loss rate, interarrival jitter, roundtrip delay • Explicit leave (BYE) • Compound packets (SDES CNAME + RR)

22. RTCP traffic Control • RTP session scale: two to thousands of participants • RTCP traffic increases with session size • Want to keep to small fraction of data bandwidth (5%) • Randomized response with rate decreasing as number of participants increases • Give active senders more bandwidth • But limited by tolerable age of status

23. Single Stream Video Multicast • Adjust video output rate • Three parameters • refresh rate (?) • quantizer (color scheme 4:2:2, 4:1:1….) • movement detection threshold (what defines motion) • Application can specify which of these to control

24. Single Stream Video Multicast • RTCP is used for feedback • Feedback implosion • probabilistic probing • Fair? (No…..)

25. Replicated Streams • Destination Set Grouping • Multiple replicated streams on different multicast addresses. • Different quality and data rates. • Receivers can switch streams

26. Switching Streams • Congestion due to presence of two streams simultaneously on the same link • Bandwidth Control Protocol • Congestion History Checking before stream switch. • Local Area Bandwidth Limit restricts the number of streams received in local area.

27. Layered Video Multicast • Disjoint layers on different addresses • Cumulative subscription • Many protocols making different assumptions

28. RLM • Receiver based • Sender does not participate • Receivers share loss information • Receivers join and drop groups based on these shared loss reports. • Receivers back off when they or other receivers see congestion. • The higher the layer, the longer the back-off duration.

29. Problems with RLM • Receiver Consensus • Fast Leaves and Joins • Impact of failed experiments on topologically unrelated receivers. • UNFAIR • Arguably the most cited and most maligned protocol!!

30. TCP rate-based Congestion Control • Analyze TCP to come with a magic formula to describe • Bandwidth = 1.3 * MTU / (RTT * sqrt(Loss)) • Adapt sender rate to match such a formula. • But what is RTT? • Let receivers make this decision. • Define loss thresholds based on this formula, for each layer. • If loss exceeds this threshold, drop a layer…

31. http://www.psc.edu/networking/papers/tcp_friendly.html

32. Summary • Multicast • RTP/RTCP • Rate Adaptation • Issues: • Fairness • Intra-session • Inter-session • Stability • Deployability • Administrative Issues